(1) Field of the Invention
This invention relates to a method for controlling a slip clutch device used in a motor vehicle.
(2) Description of the Prior Art
In a conventional torque transmitting system of the hydraulic torque converter type comprising a slip clutch arranged in parallel, as shown in FIG. 8, the rotating power from the output shaft 1 of an engine E (this is the input shaft of a torque converter and clutch 7 mentioned below) is transmitted to the input shaft 4 of a transmission 3 (this is the output shaft of the torque converter and clutch 7 mentioned below) by way of a torque converter 2, changed in speed properly and transmitted to driving wheels by way of the output shaft 5 of the transmission 3 and a differential mechanism 6.
The clutch 7 is arranged in parallel with the torque converter 2 between the output shaft 1 of the engine E and the input shaft 4 of the transmission 3. In normal operation, a front cover 1a on the side of the output shaft 1 and a facing 4b provided on a clutch plate 4a on the side of the input shaft 4 are kept in an engaged state with a fine slip so that the clutch plate 4a is rotated almost at the same speed as the front cover 1a.
Under the following conditions (i.e. the following driving states or engine conditions), the front cover 1a and clutch plate 4a are brought into a released state by a clutch control mechanism M1 so that the input shaft 4 is rotated by the driving force transmitted by way of the torque converter 2:
(1) when the temperature of cooling water for the engine is below 50.degree. C.,
(2) in the forward movement at the first gear or in the backward movement,
(3) when the rotation speed of the engine is below about 1300 rpm,
(4) in the operating zone wherein the rotation speed of the engine is below about 2000 rpm and the opening degree of a throttle is large,
(5) in the engine braking, or
(6) when shifted down from the third gear to the second gear.
The clutch control mechanism M1 comprises: a control unit 13 which receives detection signals from a water temperature sensor 8, a rotation speed sensor 9 for a planetary gear part 3a, a rotation speed sensor 10 for the transmission output shaft 5, an engine rotation speed sensor 11 and a throttle opening sensor 12, which detect each of the above-mentioned conditions respectively; a duty solenoid valve 14 which receives control signals from said control unit 13 and is opened to control the control oil pressure from a pressure regulation valve 15; and an oil pressure control valve 16 which receives the control oil pressure regulated by said duty solenoid valve 14 and controls the feed and discharge of working oil from an oil pressure source (not shown) to and from the working oil chamber 7a and release oil chamber 7b of the clutch 7.
The clutch 7 is so set that a fine slipping state is caused between a complete engagement period in that the speeds of the front cover 1a and clutch plate 4a are equal to each other and a sliding state produced by the torque converter 2 (this is the state in that the clutch 7 is released), by causing the clutch plate 4a to slip finely to the front cover 1a in the engagement of the same clutch 7. This fine slip is feedback-controlled so as to become a set target value So previously set in proportion to the rotation speed and load of the engine E, with some tolerance provided to the minimum value that can interrupt the torque variation of the engine E.
In a conventional control method which is practised in such a slip clutch device as mentioned above, the absolute value S of a difference between the rotation speed of the engine and that of a turbine is compared with a target slip So, and then a corrective quantity for the duty of a clutch-engaging oil pressure per unit time (%/sec) is determined in proportion to the rate (.DELTA.S/S) of a difference .DELTA.S (=S-So) between the absolute value S and the target slip So to the absolute value S, as shown in FIG. 9.
In such a conventional method as aforementioned, which controls the slip clutch device, however, the .mu.-v characteristics (the coefficient of friction - slipping velocity characteristics) of the facing 4b would be worsened, as shown in FIG. 10, due to the deterioration of the working oil or the mixing of other oils different in kind in the course of use, although a stable operation is obtained at the beginning of use.
Because of this worsening, the feedback control system thereof would become unstable and the slip in the clutch would be disadvantageously hunting at about 1 Hz, as shown in FIG. 11 (a) and (b).